Francisella tularensis is a pathogenic species of Gram-negative coccobacillus, an aerobic bacterium.[1] It is nonspore-forming, nonmotile,[2] and the causative agent of tularemia, the pneumonic form of which is often lethal without treatment. It is a fastidious, facultative intracellular bacterium, which requires cysteine for growth.[3] Due to its low infectious dose, ease of spread by aerosol, and high virulence, F. tularensis is classified as a Tier 1 Select Agent by the U.S. government, along with other potential agents of bioterrorism such as Yersinia pestis, Bacillus anthracis, and Ebola virus. When found in nature, Francisella tularensis can survive for several weeks at low temperatures in animal carcasses, soil, and water. In the laboratory, F. tularensis appears as small rods (0.2 by 0.2 μm), and is grown best at 35–37 °C.[4]
This species was discovered in ground squirrels in Tulare County, California in 1911. Bacterium tularense was soon isolated by George Walter McCoy (1876–1952) of the US Plague Lab in San Francisco and reported in 1912. In 1922, Edward Francis (1872–1957), a physician and medical researcher from Ohio, discovered that Bacterium tularense was the causative agent of tularemia, after studying several cases with symptoms of the disease. Later, it became known as Francisella tularensis, in honor of the discovery by Francis.[5] [6] [7]
The disease was also described in the Fukushima region of Japan by Hachiro Ohara in the 1920s, where it was associated with hunting rabbits. In 1938, Soviet bacteriologist Vladimir Dorofeev (1911–1988) and his team recreated the infectious cycle of the pathogen in humans, and his team was the first to create protection measures. In 1947, Dorofeev independently isolated the pathogen that Francis discovered in 1922. Hence it is commonly known as Francisella dorofeev in former Soviet countries.
Three subspecies (biovars) of F. tularensis are recognised (as of 2020):[8]
Additionally, F. novicida[9] has sometimes previously been classified as F. t. novicida. It was characterized as a relatively nonvirulent Francisella; only two tularemia cases in North America have been attributed to the organism, and these were only in severely immunocompromised individuals.
F. tularensis has been reported in invertebrates including insects and ticks, and vertebrates such as birds, amphibians, reptiles, fish and mammals, including humans. Human infection is often caused by vectors, particularly ticks but also mosquitos, deer flies and horse-flies. Direct contact with infected animals or carcasses is another source. Important reservoir hosts include lagomorphs (e.g. rabbits), rodents, galliform birds and deer. Infection via fomites (objects) is also important. Human-to-human transmission has been demonstrated via solid organ transplantation.[10]
F. tularensis can survive for weeks outside a mammalian host and has been found in water, grassland, and haystacks. Aerosols containing the bacteria may be generated by disturbing carcasses due to brush cutting or lawn mowing; as a result, tularemia has been referred to as "lawnmower disease". Epidemiological studies have shown a positive correlation between occupations involving the above activities and infection with F. tularensis.
Human infection with F. tularensis can occur by several routes. Portals of entry are through blood and the respiratory system. The most common occurs via skin contact, yielding an ulceroglandular form of the disease. Inhalation of bacteria, particularly biovar F. t. tularensis, leads to the potentially lethal pneumonic tularemia. While the pulmonary and ulceroglandular forms of tularemia are more common, other routes of inoculation have been described and include oropharyngeal infection due to consumption of contaminated food or water, and conjunctival infection due to inoculation at the eye.
F. tularensis is a facultative intracellular bacterium that is capable of infecting most cell types, but primarily infects macrophages in the host organism. Entry into the macrophage occurs by phagocytosis and the bacterium is sequestered from the interior of the infected cell by a phagosome. F. tularensis then breaks out of this phagosome into the cytosol and rapidly proliferates. Eventually, the infected cell undergoes apoptosis, and the progeny bacteria are released in a single "burst" event[11] to initiate new rounds of infection.
The virulence mechanisms for F. tularensis have not been well characterized. Like other intracellular bacteria that break out of phagosomal compartments to replicate in the cytosol, F. tularensis strains produce different hemolytic agents, which may facilitate degradation of the phagosome.[12] A hemolysin activity, named NlyA, with immunological reactivity to Escherichia coli anti-HlyA antibody, was identified in biovar F. t. novicida.[13] Acid phosphatase AcpA has been found in other bacteria to act as a hemolysin, whereas in Francisella, its role as a virulence factor is under vigorous debate.
F. tularensis contains type VI secretion system (T6SS), also present in some other pathogenic bacteria.[14] It also contains a number of ATP-binding cassette (ABC) proteins that may be linked to the secretion of virulence factors.[15] F. tularensis uses type IV pili to bind to the exterior of a host cell and thus become phagocytosed. Mutant strains lacking pili show severely attenuated pathogenicity.
The expression of a 23-kD protein known as IglC is required for F. tularensis phagosomal breakout and intracellular replication; in its absence, mutant F. tularensis cells die and are degraded by the macrophage. This protein is located in a putative pathogenicity island regulated by the transcription factor MglA.
F. tularensis, in vitro, downregulates the immune response of infected cells, a tactic used by a significant number of pathogenic organisms to ensure their replication is (albeit briefly) unhindered by the host immune system by blocking the warning signals from the infected cells. This downmodulation of the immune response requires the IglC protein, though again the contributions of IglC and other genes are unclear. Several other putative virulence genes exist, but have yet to be characterized for function in F. tularensis pathogenicity.
Like many other bacteria, F. tularensis undergoes asexual replication. Bacteria divide into two daughter cells, each of which contains identical genetic information. Genetic variation may be introduced by mutation or horizontal gene transfer.
The genome of F. t. tularensis strain SCHU4 has been sequenced.[16] The studies resulting from the sequencing suggest a number of gene-coding regions in the F. tularensis genome are disrupted by mutations, thus creating blocks in a number of metabolic and synthetic pathways required for survival. This indicates F. tularensis has evolved to depend on the host organism for certain nutrients and other processes ordinarily taken care of by these disrupted genes.
The F. tularensis genome contains unusual transposon-like elements resembling counterparts that normally are found in eukaryotic organisms.[16]
Much of the known global genetic diversity of F. t. holarctica is present in Sweden.[17] This suggests this subspecies originated in Scandinavia and spread from there to the rest of Eurosiberia.
See also: Tularemia. When the U.S. biological warfare program ended in 1969, F. tularensis was one of seven standardized biological weapons it had developed as part of German-American cooperation in the 1920s–1930s.[18]